EngineBackend implementation backed by Postgres.
RFC-v0.7 Wave 0 — scaffold. This crate lands the
[PostgresBackend] struct + the impl EngineBackend block with
every method stubbed to return EngineError::Unavailable.
Subsequent waves fill in bodies:
- Wave 1: cross-cutting error/helpers.
- Wave 2: describe / list / resolve (read surface).
- Wave 3: schema migrations (replaces the Wave 0 placeholder).
- Wave 4: LISTEN/NOTIFY wiring + stream reads/tails.
- Wave 5-7: hot-path write methods.
- Wave 8: ff-server wire-up + dual-backend running.
The trait stays object-safe; consumers can hold
Arc<dyn EngineBackend>. No ferriskey dep — this crate's
transport is sqlx.